1. Field of the Invention
The present invention relates to a control system for a printing machine, and more particularly, to a decentralized control system utilizing a multiple bus communication structure for a sheet-fed offset printing machine.
2. Discussion of the Related Art
Currently utilized sheet-fed offset printing machines generally have a central control system for controlling the operation of the printing machines. The central control system typically comprises a programmable controller or PC-board controller. It is also known to divide the control of the printing machine by function. Thus, a first computer can be provided which permanently reads in the switching states or signals from actuators, operating keyboards or sensors, and a second computer can be used to control the main drive and those switching processes of the printing machine which relate to the rotation angle of moving machine parts. Rotation angle dependent switching functions include, for example, the throwing-on and the throwing-off of print, the throwing-on and the throwing-off of ink and damping solution applicator rollers, the blocking of the sheet inlet, the switching of the sheet feeder unit and the switching of specific functions in the deliverer unit. These switching processes are time critical, e.g., the throwing-on of the inked blanket cylinder onto the back-pressure cylinder carrying the sheet may only take place during the channel correspondence of the cylinder, that is to say when the first sheet to be printed is already lying on the back-pressure cylinder.
A disadvantage associated with the control system described above is that the control software or program for implementing the sequence of switching functions of the printing machine must exactly correspond to the configuration of the printing machine. In other words, any change in the configuration of the printing machine would necessitate reprogramming of the control system. For example, a sheet-fed printing machine having a different number of printing units thus necessitates different programming of the corresponding control system. This situation becomes significantly more complicated as a result of the fact that nowadays there is a distinct trend towards so called in-line finishing or further processing. In the case of sheet-fed offset printing machines there are often one or more varnishing devices or other coating units interposed after the last printing unit and before the deliverer unit. Since angular position dependent switching processes also have to be executed in these devices, these functions must also be undertaken by the central control system.
DE 3 815 534 A1 discloses a central control system of the type briefly outlined above, that has a system for detecting the position of moving machine parts. For this purpose, the signals from an incremental encoder mounted on a single-turn rotating shaft of the machine are evaluated. In this system, not only are the square-wave signals, phase-shifted by 90.degree., from the incremental encoder evaluated, but also the so called zero pulses which occur in each case at one complete single-turn machine revolution. A central control system having a system of this type exhibits the above mentioned disadvantages associated therewith. Instead of an incremental encoder, an absolute angle encoder can also be used for detecting the position of the moving machine parts. However, this alters nothing in the complexity of the control system which results if a printing machine is equipped with a varying number of units.
EP 0 543 281 A1 discloses a control system for rotary printing machines, in which part of the plant is assigned a computer representing the control system, and the computers in the individual parts of the plant are connected to one another via a bus system designated as a network for the purpose of signal exchange. As the linking interface between the individual computers, ARCNET by means of coax cable is proposed. How individual rotation angle dependent switching processes are to be executed is not described in this document.
DE 4 214 394 A1 discloses a rotary printing machine which has a number of individually driven cylinders as well as a separately driven folding apparatus. The individual drives of the cylinders and the drive regulators are comprised into printing position groups at will, in which arrangement from the folding apparatus a position reference is derived, and the governing of the printing position groups occurs through a superordinated pilot system. This state of the art system relates to a drive control for achieving synchronism in the case of several individual drives.
DE 3 406 924 C2 discloses a device for the generation of control signals for a printing machine in order, by means of these control signals, to trigger switching processes at different angle positions of the printing machine cylinders or the like. The described device includes an impulse-generating arrangement and of an impulse processing arrangement engaged on an outlet side, the outputs of which deliver the control signals with variant impulse sequences and lengths. The impulse-processing arrangement contains there at least one arrangement for rotation angle address generation, the outputs of which are formed by parallel outputs of a rotation angle increment counter, and a memory contructed as a PROM with which, on each of its outputs, independently timed control signals are suppliable from the other outputs.
DE 3 836 310 A1 discloses a process and an arrangement for the control of switching processes on a printing machine in which here, over a computer along with allocated angle encoder, the switching processes are executed in several units (feeder, discharger, printing mechanisms).